Abstract: In this paper,the effects of barriers and asperities on the determination of seismic moment have been quantitatively examined by using the solution for a penny-shaped crack with non-uniform traction. It has been found that the moment of a fault with a barrier is about 40 percent of the moment on the fault with uniform stress drop and the same area,if the radius of the barrier is not very small,nor approximates to that of the fault. Therefore,it follows that,in this case,the apparent stress drop is about 40 percent of △Af=SA-f where SA is the far-field stress and f is the uniform residual frictional stress. Furthermore,a pre-existing slip zone can substantially amplify the seismic moment due to the failure of an asperity. Especially,the amplification is very large when the radius of the asperity is small. For example,the moment on the asperity is over 7 times and 30 times of the moment at the failure of a uniform shear fault with radius C,respectively,when the radius of the asperity is 20 percent and 10 percent of the radius C of the fault.The difference between three-dimensional and two-dimensional effects is compared in this paper. It is founl that Mx0/M0c is not sensitive to the change of asperity radius for three-di-mensionad faults,this differs very mith from the result with two-dimensional fault,where Mx0 is the moment due to the failure of the fault with an asperity and M0c is the moment of a uniform fault. Therefore,two-dimensional analsis is not suitable to a three-dimensional fault with asperity. However,when examining the effect of pre-existing slip zones on the moment due to failure of asperity,the difference between the results of plane strain and three-dimensional case is not apparent.